FIG. 4 shows a conventional tuner.
An input terminal 11 couples an FM intermediate-frequency signal to an FM detector 12. The output terminal of the FM detector 12 is connected to amplifiers 13 and 14, and the output of the amplifier 14 is connected to an output terminal 15.
The output terminal of the amplifier 13 is grounded through a capacitor C1 for removing an AC signal and grounded through a resistor R11 and a reference voltage Vref. Transistors Q11, Q12, Q13 constitutes a detuning mute band detecting section that is connected across the two terminals of the resistor R11. To control the gain of the amplifier 14, the collector of the transistor Q12 is connected to a mute circuit 16 through transistors Q16, Q15, and Q14 and a resistor R12. In addition, the collector of the transistor Q12 is connected through a resistor R13 to a transistor Q17 to generate a tuning detection signal. The collector of the transistor Q17 is connected to a power source Vcc through a resistor R14 and to an output terminal 17 for the tuning detection signal.
With the above arrangement, an FM intermediate-frequency signal supplied to the input terminal 11 is detected by the FM detector 12. A detection output signal is amplified by the amplifier 13 and applied to the output terminal 15 through the amplifier 14.
The AC component of the output signal from the amplifier 13 is removed by the capacitor C1, and the DC part of the output signal is supplied to the resistor R11. When a voltage generated across two terminals A and B of the resistor R11 is lower than the voltages of the transistors Q11 and Q12, i.e., .+-.V.sub.BE, the transistors Q11 and Q12 are set in an OFF state. Therefore, the transistors Q17 and Q14 driven by the transistors Q11 and Q12 are set in an OFF state. For this reason, a high-level tuning detection signal is supplied to the output terminal 17, and the mute circuit 16 is not driven.
When the voltage between the terminals A and B of the resistor 11 is higher than the voltage .+-.V.sub.BE of the transistor Q11 or Q12, the transistor Q11 or Q12 is turned on. Therefore, the transistor Q17 is turned on, and a low-level detuning detection signal is supplied to the output terminal 17. The transistor 14 is turned on to drive the mute circuit 16, and the detection output is attenuated.
In the above conventional tuning detecting circuit, the transistors Q17 and Q14 are driven by currents obtained by shunting the collector output current of the transistor Q11 or Q12 through the resistors R12 and R13. That is, the currents shunted by the resistors R12 and R13 are used to detect tuning and detuning and to drive the mute circuit 16. For this reason, when a ratio of a tuning detection band to a non mute band is set to be a predetermined ratio, e.g., if the resistance of the resistor R13 is decreased, the tuning detection band is narrowed, and the non-mute band is widened. Therefore, the band ratio cannot be easily set.
In addition, since an impedance from the resistor R13 to the transistor Q17 is different from an impedance from the resistor R12 to the transistor Q15, the band ratio is not easily set by a ratio of the resistor R13 to the resistor R12. The band ratio is influenced by a current amplification factor .beta.,i.e., a base current value of the transistor Q17. For example, when the transistor Q17 is set in an ON state, the base current value is changed by the value of the resistor R14 for controlling the collector current of the transistor Q17. Therefore, the band width is influenced, and the band ratio is changed.